Digital watermark embedding method, digital watermark extracting method, recording medium, image recording device, and image replaying device

ABSTRACT

Embedding data to be embedded as a digital watermark is acquired. Packing data is formed in which the embedding data is repeatedly connected three times or more sequentially without interval. Real embedding information is formed such that a redundancy bit with a fixed length that is used for an error correction of an information bit is added immediately after the information bit in which the packing data is subdivided into data each having a fixed length. The real embedding information is embedded into the image data itself. The embedding mechanism for the information is not easily understood, and security level is high.

BACKGROUND OF THE INVENTION

This invention relates to a digital watermark embedding method and itsrelated art that are capable of improving the evidential reliability ofdigital image data.

With the spread of digital still cameras and video cameras, in additionto conventional and widely used silver bromide photography, digitalimage data (hereinafter designated simply “image data”) is now easy touse.

Image data can be input into an information-processing device, such as apersonal computer, can be edited using image processing software. Forexample, part of the image data can be cut off and replaced with otherimages. With digital image processing, a level is reached in which eventhe eyes of a professional are unable to discover whether an image is anedited image or an entirely original image.

On the other hand, it is very difficult to edit an image taken withsilver bromide photography. In other words, the probability is very lowthat objects on a silver bromide photograph are falsified. Accordingly,it may be said that the silver bromide photography has high credibilityand high evidential reliability.

In contrast, it must be said that digital image data can be easilyfalsified by editing. Accordingly, the evidential reliability of adigital image is low in the absence of measures to improve thecredibility of the digital image data.

Cases where the characteristics of the image data are abused nevercease. For example, let it be supposed that an offender X hasmaliciously falsified image data for the purpose of casting aspersionson a person A. First, the offender X prepares an undesirable base image(for example, an image of violent or obscene scenes) containing an imageof a person A. Then the offender X substitutes only a face part ofanother person on this base image for the face of the person A.Thereafter, the offender X posts this image on a web site of theInternet to slander the person A in support of a false story about theperson A. Besides such defamatory activity, other crimes or unfair actsof falsifying image data are now made possible by the ease with which adigital image can be edited.

In response to this problem, there are demands for a technique capableof judging whether image data has been falsified or not, and capable ofpreventing the falsification beforehand, and, additionally, capable ofimproving the evidential reliability of digital image data.

As a solution thereto, a technique in which a digital signature isattached to image data can be mentioned (see Japanese Unexamined PatentPublication Nos. Hei-11-215452 and Hei-11-308564), the disclosure ofwhich is hereby incorporated by reference.

However, in light of these references, the digital signature can beeasily removed from the image data. With the digital signature removed,a judgment on falsification becomes impossible. Further, the amount ofimage data is increased by the data of the digital signature. As aresult, when the amount of digital data storage is limited, the numberof sheets of images that can be recorded is reduced.

Further, in most cases, image data is irreversibly compressed andencoded, before recording. Because the image data before compression isdifferent from the image data that has been compressed, if a digitalsignature is applied to image data before compression/encoding, thedigital signatures on the original and the compressed image data in thetwo cases are different whether or not the digital data has beenfalsified. For this reason, if the image data is compressed, the digitalsignature is applied to image data after compression/encoding.

If the image data is falsified before compression, a decision on whetheror not falsification has occurred cannot be based on the digitalsignature. Further, each time compression/extension processing isrepeated, a new digital signature must be added to the compresseddigital data. This is not practical.

Thus, the technique of attaching a digital signature to image datadoesn't give sufficient evidential reliability to the image data.

In consideration of the foregoing, the idea of embedding a digitalwatermark into image data itself and making a judgment on falsificationon the basis of this digital watermark can be proposed as a technique inwhich an error rate is reduced to a negligible extent in spite of thefact that compression/extension is performed or errors occur in datatransmission. Additionally, the following two respects are neededpractically.

(A) The embedding mechanism of the digital watermark is not easilyunderstood. If the embedding mechanism is easily understood, there is achance that the digital watermark itself might be falsified, with aresulting reduction in its credibility.

(B) The amount of data to be embedded should be as small as possible.This is for economical efficiency in information processing.

Referring to FIG. 6, as disclosed in Japanese Unexamined PatentPublication No. Hei-11-85550, the point hereof will be hereinafterdescribed in detail. It is assumed that an embedding data length is 32bytes (256 bits). A code term length is 31 bits containing 21information bits and 10 redundancy bits for error correction.

First, when recording with an embedded digital watermark, the followingsteps are executed.

(1) The embedding data (256 bits) is subdivided for each information bit(21 bits). Herein, the end of the embedding data situated at the 13thcode term has only four bits. Four bits is smaller than the number ofinformation bit (21 bits). Padding (dummy data) of either “1” or “0” isapplied to all the remaining bits (21−4=17 bits).

(2) Thereafter, redundancy bits (10 bits) of the 1st to 13th code termsare added. Then 1st to 13th code terms after the redundancy bits areadded are brought together into one as unit data.

(3) Thereafter, information obtained by repeating the unit data threetimes for a later majority decision (three sets in the total of the 1stto 13th code terms, 14th to 26th code terms, and 27th to 39th codeterms) is defined as real embedding information.

(4) The real embedding information is embedded into image data, and itis stored on a recording medium.

Next, when reproduced (i.e., when the digital watermark is extracted),the following steps are executed.

(a) The image data and the real embedding information are separated andextracted from the recording medium.

(b) Based on the image information, reproduction is carried out.

(c) The real embedding information is divided into three parts, and thesame unit data, repeated three times, are extracted.

(d) The error correction of a corresponding information bit is carriedout by a redundancy bit.

(e) A majority decision is made regarding the corresponding bit of eachunit data that has been extracted, and error correction is performed.For example, a majority decision regarding the first bit of theinformation bit is made by each first bit of the 1st, 14th, and 27thcode terms.

(f) The result of the majority decision is used as embedding data.

Indeed, according to this procedure, the error rate is reduced to anegligible extent in spite of the fact that compression/extension isperformed or errors occur in data transmission.

However, the two respects of (A) and (B) mentioned above are notsatisfied. That is, since the same unit data is simply repeated threetimes, high regularity is exhibited, and therefore the embeddingmechanism is easily understood. For this reason, there is a fear thatthe embedded digital watermark itself will be falsified, and therecorded digital data still lacks sufficient evidential reliability.

Further, since dummy data that has been subjected to padding many timesappears, there is much useless labor, and this is disadvantageous fromthe viewpoint of economical efficiency in information processing.

OBJECTS AND SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a digitalwatermark embedding method and its related art that are capable ofgiving sufficient evidential reliability to image data and capable ofreducing useless labor.

Briefly stated, the present invention provides a system in whichembedding data to be embedded as a digital watermark is acquired.Packing data is formed in which the embedding data is repeatedlyconnected three times or more sequentially without interval. Realembedding information is formed such that a redundancy bit with a fixedlength that is used for an error correction of an information bit isadded immediately after the information bit in which the packing data issubdivided into data each having a fixed length. The real embeddinginformation is embedded into the image data itself. The embeddingmechanism for the information is not easily understood, and securitylevel is high.

According to an embodiment of the invention, there is provided a digitalwatermark embedding method comprising: acquiring embedding data to beembedded as a digital watermark, forming packing data in which theembedding data is repeatedly connected at least three times to besequential without interval, forming real embedding information in whicha redundancy bit with a fixed length that is used for error correctionof an information bit is added immediately after the information bit inwhich the packing data is subdivided into data each having a fixedlength, and embedding real embedding information into image data.

According to a feature of the invention, there is provided a recordingmedium for recording an image comprising: means for acquiring embeddingdata to be embedded as a digital watermark, means for embedding thedigital watermark as real embedding information, means for addingpacking data in which the embedding data is repeatedly connected atleast three times sequentially without interval, means for adding aredundancy bit with a fixed length that is used for error correction ofan information bit immediately after the information bit, and means forsubdividing the packing data into data each having a fixed length.

According to a further feature of the invention, there is provided animage recording device comprising: embedding data input means foracquiring embedding data to be embedded as a digital watermark, packingdata forming means for forming packing data in which the embedding datais repeatedly connected at least three times sequentially withoutinterval, redundancy bit addition means for forming real embeddinginformation in which a redundancy bit with a fixed length that is usedfor error correction of an information bit is added immediately afterthe information bit in which the packing data is subdivided into dataeach having a fixed length, embedding means for embedding the realembedding information into image data concerned, and output means forwriting information onto a recording medium on the basis of the imagedata concerned in which the real embedding information is embedded.

According to a further feature of the invention, there is provided animage replaying device comprising: image signal output means foroutputting an image signal on the basis of information read from arecording medium that records an image concerned in which a digitalwatermark is embedded as real embedding information, comprising: meansfor acquiring the real embedding information containing embedding datato be embedded as a digital watermark is acquired, means for repeatedlyconnecting packing data in which the embedding data is repeatedlyconnected at least three times to be sequential without interval, andmeans for adding a redundancy bit with a fixed length that is used foran error correction of an information bit immediately after theinformation bit in which the packing data is subdivided into data eachhaving a fixed length.

According to a first aspect of the present invention, a digitalwatermark embedding method has a step of acquiring embedding data to beembedded as a digital watermark, a step of forming packing data in whichthe embedding data is repeatedly connected at least three times insequence without any interval, a step of forming real embeddinginformation in which a redundancy bit with a fixed length that is usedfor error correction of an information bit is added immediately afterthe information bit in which the packing data is subdivided into data,each having a fixed length, and a step of embedding the real embeddinginformation into the image data.

With this structure, the error rate is reduced to a negligible extent inspite of the fact that compression/extension is performed or errorsoccur in data transmission.

Additionally, in general, since packing data is used, code terms areentirely different data from each other, and its regularity is low. Itis therefore very difficult to understand the embedding mechanism of thedigital watermark. Additionally, since the place where padding occurs islimited only to one place in the last code term in the entire realembedding information, embedding can be carried out without addingunnecessary wasteful terms.

According to a second aspect of the present invention, the embeddingdata is enciphered in addition to the feature of the first aspect of thepresent invention.

With this structure, even if an embedded digital watermark is extractedby chance, the contents and meaning thereof can be designed not to beunderstandable. Therefore the safety and evidential reliability of theimage data is improved even more.

According to a third aspect of the present invention, the real embeddinginformation is manifoldly embedded after being interleaved in additionto the feature of the first aspect of the present invention.

In general, there is a weak point in a burst error if a BCH code is usedfor a redundancy bit. However, according to this structure, burst errorsare sufficiently dealt with.

The above, and other objects, features and advantages of the presentinvention will become apparent from the following description read inconjunction with the accompanying drawings, in which like referencenumerals designate the same elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an image recording/reproducing deviceaccording to an embodiment of the present invention.

FIG. 2 is a data structure diagram of real embedding informationaccording to the embodiment.

FIG. 3 is a flowchart showing the embedding process of a digitalwatermark according to the embodiment.

FIG. 4 is a flowchart showing the extracting process of the digitalwatermark.

FIG. 5 is a flowchart showing a falsification judging process.

FIG. 6 is a data structure diagram of conventional real embeddinginformation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, an embodiment of the present invention is made upof three systems. A recording system is represented above the brokenline L1 of FIG. 1. A reproducing system is represented below the brokenline L2. A control system is represented between the broken lines L1 andL2. Parts of the present invention may be formed by omitting one of thereproducing-only image replaying device or the recording-only imagerecording device.

(Recording System)

An embedding-data input means 1 inputs embedding data to be embedded asa digital watermark into image data. Information represented by thisdigital watermark can be arbitrarily selected, as a matter of course.The digital watermark information may be information on, for example, adevice name by which a photograph is taken, a date, a place, or asurrounding sound.

Alternatively, the embedding data input means 1 itself may generate thedigital watermark, or a user may input it.

Preferably, the embedding data input means 1 encrypts the embedding dataaccording to conventional cryptography, in order to improve security.For example, when the United States cryptographic standard DES is used,encrypting is carried out using 56 bits as one block.

Referring now also to FIG. 2, a packing data generation means 2generates packing data in which the embedding data input from theembedding data input means 1 are repeatedly connected at least threetimes in sequence without interval (preferably, an odd number times toavoid an indefinite majority decision).

In this example, an embedding data length is assumed to be 32 bytes (256bits), with a code term length of 31 bits (wherein information bits are21 bits, and redundancy bits for error correction are 10 bits), as inFIG. 6.

Further, in this example, since the end of the packing data terminatesat the 12th bit of the information bits of the 37th code term, paddingis applied to the remaining information bits (21−12=9 bits) of the 37thcode term.

Without being limited to this example, the present invention can belikewise applied to an example in which other code terms are used, as amatter of course.

Referring now also to FIG. 2, a redundancy bit addition means 3 addsredundancy bits (10 bits) at the end of the information bits (21 bits)for error correction of the front information bits about each code term.Real embedding information is output.

Herein, this redundancy bit method is arbitrary. For example, a hammingcode method, a BCH code method, or a Reed-Solomon method can befollowed.

Since BCH (31,21) is used in this embodiment, 10 bits as errorcorrection bits are added to the encrypted data of 21 bits.

An image data input means 4 inputs image data that has been output from,for example, an image pickup means (still or moving picture pickupmeans) for outputting a digital image signal. As a matter of course, animage-recording device of the present invention can be constructedintegrally with this image pickup means.

A pixel block division means 5 outputs image data output originating inthe image data input means 4 in the form of one image, or outputs theimage data in the form of pixel blocks each having a predetermined sizethat are obtained by dividing a single image.

An embedding means 6 embeds real embedding information as a digitalwatermark into the image data input from the pixel block division means5. The embedded image data is output to a compression/modulation means7. As a result, the digital watermark is integrally treated the same asthe image data itself, and not as an attachment to the image data. It isallowable to embed the digital watermark under any well-known method.

Since the BCH code has a weak point with respect to burst errors, it ispreferable to prevent burst errors in such a way that a redundancy bitis added, and multi output is then carried out with interleaves.

The compression/modulation means 7 compresses and modulates the embeddedimage data input from the embedding means 6, and outputs it to a firstterminal of a switching means 8. This method preferably employs MPEG,for example, for a moving picture, and employs JPEG for a still image.

In FIG. 1, a DVD is used as a recording medium 12. Instead, CD-ROMs,DVCs, hard disks, or MOs can be used as the recording medium 12. Sincethe DVD is used in the device of this embodiment, a driving systemincludes a spindle motor 11 for rotating the recording medium 12 and apickup portion 13 serves as an input/output means for reading andwriting information in the face of the recording medium 12.

(Control System)

A control means 9 controls each element shown in the figure, andswitches the switching means 8 between read and write modes. In the readmode, the switching means 8 connects the pickup portion 13 through asecond terminal to an extension/demodulation means 14 of a reproducingsystem. In the write mode, the switching means 8 connects the output ofthe compression/modulation means 7 through the first terminal to thepickup portion 13.

(Reproducing System)

The extension/demodulation means 14 extends and demodulates theinformation read by the pickup portion 13, and outputs the embeddedimage data to a real embedding information detection means 15. Theextension/demodulation means 14 further outputs the embedded image datato an image signal output means 21, and the image signal output means 21outputs the image signal to a display device. Unlike this embodiment,display may be made on the basis of an image in which the real embeddinginformation is excluded from the embedded image data. Further, theimage-reproducing device of the present invention can be constructedintegrally with this display device.

The real embedding information detection means 15 applies processingopposite to the processing of the embedding means 6 to the embeddedimage data input from the extension/demodulation means 14, and thenextracts real embedding information, and outputs it to an errorcorrection means 16.

As shown in FIG. 2, the error correction means 16 performs errorcorrection of the information bits (21 bits) situated at the front ofthe redundancy bits on the basis of the redundancy bits (10 bits). Whileperforming error correction, the error correction means 16 calculates anerror rate, and stores the obtained error rate in a first area of anerror rate record means 17 as a first error rate.

Based on the real embedding information that has undergone errorcorrection by the error correction means 16, a majority decision means18 executes a majority decision regarding each corresponding bit of theinformation bit, and makes an error correction according to the majoritydecision. At this time, the majority decision means 18 calculates anerror rate, and stores the obtained error rate in a second area of theerror rate record means 17 as a second error rate.

In this embodiment, each corresponding bit of the information bits isseparated by 256 bits when paying attention only to the information bit,as shown in FIG. 2. For example, in order to make a majority decisionregarding the first bit of the information bit, the majority decision isexecuted at the 1st bit of the 1st code term, at the 5th bit of the 13thcode term, and at the 9th bit of the 25th code term.

Further, referring to the first and second areas of the error raterecord means 17, the error rate calculation means 19 calculates a totalerror rate from the first and second error rates. The calculation methodfor the error rate may use the sum total simple.

When the calculation of the error rate calculation means 19 iscompleted, a falsification judgment means 20 compares the error ratecalculated by the error rate calculation means 19 with a predeterminedthreshold (which is empirically determined). If the error rate exceedsthe threshold, the falsification judgment means 20 judges that afalsification exists. If not, it judges that no falsification exists.The judgment result is returned to the control means 9.

Next, the embedding process of a digital watermark is described withreference to FIG. 3. First, in step 1, the embedding data input means 1acquires embedding data. Based on the embedding data, the packing datageneration means 2 forms packing data (step 2). Thereafter, in step 3,the redundancy bit addition means 3 adds each redundancy bit to formreal embedding information.

In step 4, the embedding means 6 embeds the real embedding informationinto the image data that has passed through the image data input means 4and the pixel block division means 5. The compression/modulation means 7compresses and modulates the result, and thereafter the pickup portion13 stores data on the recording medium 12 (step 5).

Next, the extracting process of the digital watermark is described withreference to FIG. 4. First, in step 10, the pickup portion 13 readsinformation from the recording medium 12, and the extension/demodulationmeans 14 extends and demodulates it. Thereafter the information is inputto the real embedding information detection means 15. In step 11, thereal embedding information detection means 15 extracts real embeddinginformation. The error correction means 16 then makes an errorcorrection using the redundancy bit (step 12). The majority decisionmeans 18 then makes an error correction according to the majoritydecision on each corresponding bit of the information bit (step 13). Thereal embedding information is extracted and output to the control means9 (step 14).

Next, a falsification judgment process is described with reference toFIG. 5. First, when the first and second error rates are stored on theerror rate record means 17, the error rate calculation means 19calculates a total error rate in step 21, and outputs the result to thefalsification judgment means 20. In step 22, the falsification judgmentmeans 20 compares the error rate calculated by the error ratecalculation means 19 with a predetermined threshold. If the error rateexceeds the threshold, the falsification judgment means 20 judges that afalsification exists (step 23). If the error rate is lower than thethreshold, the falsification judgement means 20 judges that nofalsification exists. (step 24).

As is apparent from a comparison between FIG. 2 and FIG. 6, the realembedding information of the prior art in FIG. 6 is 1209 bits and, incontrast, the real embedding information of FIG. 2 is only 1147 bits.Accordingly, it will be understood that a saving of 5% or more isrealized in the present invention compared to the prior art in spite ofthe fact that the code term lengths or the lengths of the embedding dataare identical to each other.

Further, it is a noteworthy fact the prior art system of FIG. 6 usesthree sets of padding, whereas the present invention in FIG. 2 uses onlyone. Accordingly, efficient processing can be carried out in the presentinvention.

Further, the same unit data is monotonously repeated in FIG. 6, whichmeans high regularity. However, in FIG. 2, such unit data does notexist, and regularity is low. The low regularity adds complexity to oneattempting to decode the information.

Having described preferred embodiments of the invention with referenceto the accompanying drawings, it is to be understood that the inventionis not limited to those precise embodiments, and that various changesand modifications may be effected therein by one skilled in the artwithout departing from the scope or spirit of the invention as definedin the appended claims.

1. A digital watermark embedding method comprising the steps of:acquiring embedding data to be embedded as a digital watermark; formingpacking data in which said embedding data is repeatedly connected atleast three times to be sequential without interval; forming realembedding information in which redundancy bits with a fixed length thatare used for error correction of information bits are added immediatelyafter said information bits in which said packing data is subdividedinto data each having a fixed length; and embedding real embeddinginformation into image data.
 2. The digital watermark embedding methodof claim 1, further comprising the step of encrypting said embeddingdata.
 3. The digital watermark embedding method of claim 1, furthercomprising the steps of: interleaving said real embedding information;and thereafter multiplexedly embedding said real embedding information.4. A digital watermark extracting method for extracting a digitalwatermark from image data into which said digital watermark is embeddedaccording to the digital watermark embedding method as set forth inclaim 1, the digital watermark extracting method comprising the stepsof: extracting real embedding information from said image data; cuttingreal embedding information into code terms having a fixed length; makingan error correction of information bits located at a front of redundancybits using redundancy bits located at an end of the code term length,and thereby obtaining information bits that have undergone errorcorrection; executing a majority decision for each corresponding bit ofthe obtained information bits, and making an error correction bymajority decision; and treating said information bits that haveundergone said error correction by said majority decision as dataembedded in an image.
 5. A recording medium for recording image data,said recording medium including a data structure stored thereon, saidimage data being formed from embedding data into an original image dataas a digital watermark, said data structure comprising: an area forstoring subdivided data, said subdivided data being formed fromsubdividing packing data into data each having a fixed length, thepacking data being formed from repeatedly connecting said embedding dataat least three times sequentially without interval; and an area forstoring redundancy bits with a fixed length that are used for errorcorrection of information bits; wherein said area for storing saidredundancy bits is arranged immediately after said area for storing saidsubdivided data.
 6. The recording medium of claim 5, wherein saidembedding data is encrypted.
 7. The recording medium of claim 5, whereinsaid embedding data is interleaved.
 8. An image recording devicecomprising: embedding data input means for acquiring embedding data tobe embedded as a digital watermark; packing data forming means forforming packing data in which said embedding data is repeatedlyconnected at least three times sequentially without interval; redundancybit addition means for subdividing said packing data into subdivideddata, each having a fixed length, and thereafter adding redundancy bitswith a fixed length that are used for error correction of informationbits immediately after said subdivided data each having the fixed lengthto output real embedding information; embedding means for embedding saidreal embedding information into image data; and output means for writinginformation onto a recording medium on the basis of said image data inwhich said real embedding information is embedded.
 9. The imagerecording device of claim 8, further comprising means for encryptingsaid embedding data.
 10. The recording medium of claim 8, furthercomprising means for interleaving said real embedding information, andthereafter multiplexedly embedding said real embedding information. 11.The image recording device of claim 8, further comprising image datainput means for outputting said image data.
 12. The image recordingdevice of claim 11, wherein said image data input means includes imagepickup means.
 13. An image replaying device comprising image signaloutput means for outputting an image signal on the basis of informationread from a recording medium that records an image concerned in which adigital watermark is embedded as real embedding information, comprising:means for acquiring said real embedding information containing embeddingdata to be embedded as a digital watermark; means for repeatedlyconnecting packing data in which said embedding data is repeatedlyconnected at least three times sequentially without interval; and meansfor subdividing said packing data into subdivided data, each having afixed length, and thereafter adding redundancy with a fixed length thatare used for error correction of information bits immediately after saidsubdivided data each having the fixed length to output real embeddinginformation.
 14. The image replaying device of claim 13, furthercomprising means for encrypting said embedding data.
 15. The imagereplaying device of claim 13, further comprising means for interleavingsaid real embedding information, and thereafter for multiplexedlyembedding said real embedding information.
 16. The image replayingdevice of claim 13, further comprising a display device for making adisplay on the basis of an image signal output by said image signaloutput means.
 17. The image replaying device of claim 13, furthercomprising: real embedding information detection means for extractingreal embedding information embedded in information read from saidrecording medium on the basis of said information; error correctionmeans for making an error correction by said redundancy bits withrespect to said real embedding information; and majority decision meansfor executing a majority decision for each corresponding bit of saidinformation bits with respect to said real embedding information thathas been corrected by said error correction means, and making an errorcorrection by said majority decision.
 18. The image replaying device ofclaim 17, further comprising: error rate calculation means forcalculating an error rate of image data concerned with reference to anerror rate in said error correction by said redundancy bits and an errorrate in said error correction by majority decision; and falsificationjudgment means for comparing an error rate of image data concernedcalculated by said error rate calculation means with a predeterminedthreshold, and, if the error rate of image data concerned exceeds saidthreshold, judging that a falsification exists, and, if not, judgingthat no falsification exists.